What could be the problem?

EVNFSTR

MEMBER
1997 CHEVROLET SILVERADO
160,000 MILES

I have a 1997 K1500 with a new 5.7L Vortec 5 speed transmission that had 0 miles on both when I installed them a couple months ago. I bought the truck as a roller and never heard the old motor run so I am not sure if these problems were present before. I am having multiple problems with this truck and trying to get it running like it should. It has a surge at idle when I first start the truck for about 10 seconds it fluctuates between 500-1,000 rpm and then levels out at 600-700. It is fine until I start driving it then the throttle will rev back up to 1,000 like the cruise is stuck or the throttle is stuck open. I have replaced the IAC, and TPS. I installed an aftermarket gauge column with the oil pressure and another for the coolant temperature. When I put the new motor in I put new plugs, wires, cap, and rotor. I am running it with the cats still and some aftermarket short tube headers and a 2 1/2 exhaust with flowmasters. The other issue I have is intermittent and feels like it is loosing fuel pressure but I tested it with a gauge and it reads 52PSI but it also was not acting up at the time. I could be driving down the road and it will start bucking like crazy but very intermittantly. I would think if it were the cap and rotor it would act like that all the time and the same goes with the plugs and wires. I consider myself to be very mechanically savvy but this one has me stupmed. I have even gone to a frind of mine and we hooked a brick up to it and he does not se any codes that would help him pinpoint what the problem is. Thank You, Jeff

3 Replies

HMAC300

MECHANIC
48,601 POSTS

Your fuel pressure should be 60-66 psi with csi and 56-62with sfi. That may be part of your problem. Alsoc clean the throttleplateon both sides and the iac hole. Also your injector lines could be cracked if under the nmanifold. That could cause the up and down revving.
If� the� fuel� pressure� is� less� than� 55� psi� (379� kPa)� at� idle, � it� falls� into� the� following� 3� areas:
? The� pressure� is� regulated� but� less� than� 55� psi� (379� kPa).� The� amount� of� fuel� reaching� the� injector� is� sufficient, � but� the� pressure� is� too� low.� The� system� will� run� lean, � hard� starting� cold, � no� start, � overall� poor
performance, � and� may� set� a� DTC.� See NO START� DIAGNOSIS�.
? A� restricted� flow� causing� pressure� drop.� Normally, � a� vehicle� with� a� fuel� pressure� of� less� than� 44� psi� (300� kPa)� at� idle� is� inoperable.� However, � if� the� pressure� drop� occurs� only� while� driving, � the� engine� will
normally� surge� then� stop� running� as� the� pressure� begins� to� drop� rapidly.� This� is� most� likely� caused� by� a� restricted� fuel� line� or� plugged� fuel� filter.